Sulfur occurs in many industrial, or process streams, generally as hydrogen sulfide or as an organosulfur compound, or compounds. For example, in virgin naphtha and heating oil, the mercaptans and disulfide types are the predominant forms of sulfur. Thiophenes and benzothiophenes appear the predominant forms of sulfur in virgin gas oils and cracked stocks. A hydrotreated product, even after most of the sulfur has been removed, generally yet contains some of the native organosulfur compounds, or different forms of organosulfur compounds, and additionally hydrogen sulfide. Natural gas, flue gas, waste gas, and recycle gas streams often contain various forms of sulfur, e.g., hydrogen sulfide. Adsorbents, or sorbents, of various types have been long known as materials useful for the removal of sulfur from process streams.
The quality of these various sorbents for the adsorption and removal of sulfur varies considerably, and in many applications it is necessary to scrub essentially all of the sulfur from the process streams. This is necessary for process reasons, as well as environmental reasons. Sulfur, for example, is a well known catalyst poison which finds its way into a catalytic process employed to treat hydrocarbons, as in petroleum refining operations, principally via the feed, and it can gradually accumulate upon and poison a catalyst. Essentially all petroleum feeds as well as gas-liquid products, and recycle streams of hydrogen-containing gas contain sulfur. Most of the sulfur, because of this adverse effect, is removed from the feed, typically by hydrotreating or hydrodesulfurization. Additional sulfur removal can be achieved by passage of the hydrofined or hydrodesulfurized feed through a guard chamber or sulfur trap, e.g., by contact in a guard bed, or sulfur trap with a nickel or cobalt adsorbent.
Catalytic reforming, or hydroforming, is a well-known and important process employed in the petroleum refining industry for improving the octane quality of naphthas and straight run gasolines. This process is illustrative of one wherein the presence of sulfur can have a detrimental effect. In a typical reforming process, a series of reactors are provided with fixed-beds of sulfided catalyst which are sequentially contacted with a naphtha feed, and hydrogen, each reactor being provided with a preheater, or interstage heater, because the reactions which take place are endothermic. The more recently developed processes of this type employ poly-metallic platinum-containing catalysts (wherein one or more additional metals are added as promoters to the platinum), and it has, in fact, become essential to reduce the feed sulfur to only a few parts per million parts by weight (wppm) of feed, because of the extreme sulfur sensitivity of these catalysts. For example, in the use of platinum-rhenium catalysts it is generally necessary to reduce the sulfur concentration of the feed naphtha well below about 10 wppm, and preferably well below about 2 wppm, or even 0.5 wppm, to avoid excessive loss of catalyst activity and C.sub.5.sup. + liquid yield.
In the past, sulfur has been removed from the recycle hydrogen stream by the use of guard chambers filled with metal oxides, e.g., zinc oxide, cobalt oxide, nickel, or the like. In Paul E. Eberly, Jr.'s U.S. Pat. No. 4,263,020, in particular, there is disclosed the use of a form of metal aluminum spinel which is particularly effective for the removal of sulfur from process streams, especially for the removal of sulfur from a sulfur-containing hydrogen recycle stream as employed in reforming operations. A stoichiometric oxide spinel is described generally as a material having the formula MN.sub.2 O.sub.4 and a crystalline structure of the type exhibited by such minerals as gahnite (ZnAl.sub.2 O.sub.4), hercynite (FeAl.sub.2 O.sub.4), chromite (FeCr.sub.2 O.sub.4), or spinel (MgAl.sub.2 O.sub.4), the latter of these being the material after which the spinel structure is named. The metal alumina spinel therein described, in any event, is employed within a guard chamber as a particulate mass, and it is characterized as MAl.sub.2 O.sub.4 wherein M is chromium, iron, cobalt, nickel, copper, cadmium, mercury, or zinc; and zinc alumina spinel is described as a preferred species of sulfur adsorbent. After adsorption of the sulfur, the sulfur can be readily desorbed, or removed from said particulate mass of metal alumina spinel by contacting, and purging same with a relatively clean gas stream, suitably hydrogen, hydrogen-containing gas, or an inert gas, at elevated temperature.
The Eberly process admirably achieves its intended objective, especially in its use as an adsorbent for the removal of sulfur from process gas streams, notably sulfur-containing hydrogen gas streams, particularly hydrogen recycle gas streams as employed in reforming operations. Sulfur is readily removed from such streams sufficient to maintain sulfur in low concentrations, concentrations well below about 2 wppm, and the capacity of the adsorbent for removing sulfur in this manner is outstanding.